Equirectangular sky texture in Blender Internal?
To make equirectangular texture enhance your scene in Blender Internal, create a texture for World Background and add your map in there.


Open World scroll, check Real Sky. Without this setting texture still will work but the view won't change if rotating camera. Then go to Textures tab to create a new texture for the world.
In the new texture properties leave it on Image or Movie, open equirectangular map.
In the Mapping scroll change coordinates to Equirectangular.
In the Influence tab uncheck Blend and choose Horizon. It's rather handy to look at Preview World scroll while changing these, as it will change according to steps done. 

 

Note that as mentioned by @Mentalist in order to see your equirectangular map in the render you should also set Alpha Mode to Sky instead of Transparent in Render > OpenGL Render Options (if it was changed before).


If you'd like to make the map affect the scene lighting, enable Environment Lighting and set it to Sky Texture.




Also according to the post on BA, supporting of equirectangular maps in BI has been added rather long ago.